chore: move Drizzle env-var validation off the import seam
Replace import-time throws in drizzle.config.ts with empty-string
defaults so pnpm check / svelte-check no longer crash for forks
without Cloudflare credentials set. Validation moves to
scripts/db-check.js, which is chained into db:push, db:generate,
and db:migrate so the failure happens with a clear message only when
the env vars are actually needed.
Closes #4
https://claude.ai/code/session_016eNQzUGcKjUPx33etijhGK